The present invention relates generally to the application of a solid lubricant on steel strip and more particularly to the combination of a lubricant application procedure in-line with a method for processing steel strip.
When a steel strip is subjected to a fabricating operation, such as stamping or drawing, it is desirable for the steel strip to have a film of lubricant thereon, as that facilitates the fabricating operation. Generally, the lubricant film can be either solid or liquid, but solid lubricant films have advantages over liquid films, and solid lubricant films are preferred.
When a solid lubricant is applied to steel strip, the lubricant is applied in the form of a liquid, either in molten form or as a solution. The solid lubricant application procedure can be in-line or external. In-line application refers to application during processing of the steel strip in the steel mill, as distinguished from external application in which the solid lubricant is applied at an external processing line separate and apart from a steel mill processing line. External application requires transportation and handling (and the equipment and manpower associated therewith) over and above that employed with in-line processing, thereby substantially increasing the time, effort and money involved.
There are certain problems which must be overcome should one attempt to retrofit existing steel mill strip processing lines to incorporate in-line application of solid lubricant. One cannot merely incorporate a conventional external application procedure into an existing steel mill processing line. This is because a conventional external application line can occupy a relatively large amount of space which is not available in existing steel mill strip processing lines. Moreover, in existing steel mill strip processing lines, the steel strip moves along the processing line at a relatively rapid speed, and in-line solid lubricant application equipment desirably should be synchronized to operate at the same speed. A solid lubricant application procedure not only incorporates the step of applying the solid lubricant to the surface of the steel strip, but also it generally incorporates a thermal treatment step for the steel strip either upstream or downstream of the solid lubricant-applying step, or both. In-line application of solid lubricant requires incorporating all of these processing steps (and the equipment for performing them) into the steel mill strip processing line, while (1) accommodating those steps and equipment to the space limitations in the steel mill strip processing line, (2) synchronizing the speed of the lubricant application procedure to the relatively rapid speed at which the steel mill strip processing line is operated and (3) allowing enough flexibility in the design of the solid lubricant application equipment to accommodate both hot melt and solution-based solid film lubricants.
Examples of solid lubricants for steel strip and of procedures for applying the solid lubricant are described in Coduti, et al., U.S. Pat. No. 4,999,241 (wax type solid lubricant), Robbins, et al., U.S. Pat. No. 5,151,297 (acrylic polymer solid lubricant) and Sech U.S. Pat. No. 4,753,743 (wax type solid lubricant); the disclosures in all of these U.S. patents are incorporated herein by reference.
The Coduti, et al. patent teaches preheating a steel strip to which a wax lubricant is to be applied in molten form; the strip preheat temperature is 5.degree. to 10.degree. F. above the melting point of the wax. (The melting point for wax lubricants is typically in the range 120.degree.-150.degree. F. (49.degree.-66.degree. C.)). The higher the strip preheat temperature, the more difficult it is to accommodate the strip preheating step (and any subsequent cooling step) (a) to the space limitations in the steel strip processing line and (b) to the strip speed synchronizing requirement, imposed upon the lubricant application procedure when one attempts to combine that procedure in-line with a method for processing steel strip in an existing strip processing line.